1. Technical Field
The present invention relates to a semiconductor device capable of lowering the ON-resistance and of elevating the avalanche resistance of a transistor.
2. Related Art
A vertical transistor has been known as one type of high-voltage power MOS field effect transistor (power MOSFET). Important characteristics of the power MOSFET include lowering of the ON-resistance and elevation of the breakdown resistance. One known structure capable of reconciling these requirements is exemplified by super-junction structure (referred to as “SJ structure”, hereinafter), described in Proceedings of the 19th International Symposium on Power Semiconductor Devices & IC's, P. 37, 2007.
FIG. 14 is a plan view illustrating an exemplary configuration of a transistor having the SJ structure, and FIG. 15 is a sectional view taken alone line A-A′ in FIG. 14. The transistor has a first-conductivity-type semiconductor substrate 500, an epitaxial layer 510, base regions 520, trench gate electrodes 530, source regions 540, column regions 550, an insulating film 600, source electrode(s) 610, and drain electrode(s) 620. The epitaxial layer 510 is formed over the surface of the semiconductor substrate 500, has a first conductivity type, and functions as an electric field moderating layer. Each base region 520 has a second conductivity type, and is formed over the surface of the epitaxial layer 510. Each trench gate electrode 530 is buried in the surficial portion of the epitaxial layer 510. A gate insulating film is provided between each trench gate electrode 530 and the epitaxial layer 510. Each source region 540 has a first conductivity type, and is formed in each base region 520. Each column region 550 has a second conductivity type, and is formed in the epitaxial layer 510, more specifically in a region thereof below each base region 520. In a plan view, the trench gate electrodes 530 and the column regions 550 are linearly extended in parallel with each other.
Each source electrode 610 is formed over the epitaxial layer 510, and is connected to the base regions 520 and the source regions 540. The source electrodes 610 and the trench gate electrodes 530 are electrically isolated by the insulating film 600. The drain electrode 620 is provided on the back side of the semiconductor substrate 500.
In the semiconductor device illustrated in FIG. 14 and FIG. 15, when the transistor is turned off, having no bias voltage being applied between the trench gate electrodes 530 and the source electrodes 610, and when a reverse bias voltage is applied between the drain electrodes 620 and the source electrodes 610, a depletion layer spreads respectively at the interface between each base region 520 and the epitaxial layer 510, and at the interface between each column region 550 and the epitaxial layer 510. Since the interface between each column region 550 and the epitaxial layer 510 extends in the thickness-wise direction of the epitaxial layer 510 (vertical direction in FIG. 15), so that the depletion layer which resides at the interface spreads in the in-plane direction (transverse direction in FIG. 15) from the interface. Accordingly, the epitaxial layer 510 (including the column regions 550) is depleted over the entire region thereof, so that the breakdown voltage of the transistor is determined by the thickness of the epitaxial layer 510, irrespective of the impurity concentration of the epitaxial layer 510. The ON-resistance of the transistor may therefore be lowered by increasing the impurity concentration of the epitaxial layer 510, and the breakdown resistance of the transistor may be elevated by thickening the epitaxial layer 510.
Japanese Laid-Open Patent Publication No. 2002-076339 discloses also a configuration of a transistor having the SJ structure, in which a repetition pitch of the trenches filled with gate electrodes is made different from the pitch of arrangement of the parallel pn layers, that is, the column regions.
Japanese Laid-Open Patent Publication No. 2006-310621 discloses another SJ structure in which the column regions are provided also below the trench gates.
In a transistor having the SJ structure, the drain current of the transistor flows through the substrate (epitaxial layer, for example). Since the column regions have a conductivity type opposite to that of the substrate, the region allowing the drain current to flow therethrough will be narrowed if the occupied area of the column regions grows larger relative to the substrate, and thereby the ON-resistance of transistor may increase.
In addition, in a transistor having the SJ structure, current which generates in the process of breakdown may flow through the column region and the base region, to an electrode in contact with the base region. If the distance between the column region and the trench gate is short, the current which generated in the process of breakdown may flow through a region in the vicinity of the trench gate, and thereby the avalanche resistance may be degraded, because of thermal destruction ascribable to operation of parasitic bipolar transistors formed along the side faces of the trench gates, and dielectric breakdown ascribable to injection of carriers into the gate insulating film.